Aqueous dispersions containing ethylene copolymer waxes

ABSTRACT

Aqueous dispersions comprising at least one ethylene copolymer wax comprising from 60 to 99.5% by weight of ethylene and from 0.5 to 40% by weight of at least one ethylenically unsaturated carboxylic acid in copolymerized form and also at least one hydrophobic low molecular weight organic substance.

The present invention relates to aqueous dispersions comprising at leastone ethylene copolymer wax comprising from 60 to 99.5% by weight ofethylene and from 0.5 to 40% by weight of at least one ethylenicallyunsaturated carboxylic acid in copolymerized form and also at least onehydrophobic low molecular weight organic substance.

The preparation of aqueous dispersions of intrinsically hydrophobicsubstances is of great industrial importance. Emulsifiers are generallyused as auxiliaries for preparing and stabilizing the dispersion.However, for numerous applications, for example in paper coating, itwould be desirable to dispense with the use of emulsifiers.

EP-A 0 691 390, Progress in Organic Coatings 1995, 26, 207, and Progressin Organic Coatings 1996, 29, 201, disclose aqueous dispersions offluorescent dyes which are obtainable by free-radical polymerization of,for example, acrylic acid, methyl methacrylate, n-butyl acrylate andstyrene in an organic solvent, for example isobutanol, in the presenceof a fluorescent dye. The copolymers obtainable in this way have aweight average molecular weight M_(w) of from 20 000 to 500 000 g/mol.The solvent can be removed later, i.e. before or after the actualapplication of the dispersion. However, the use of solvents is generallydisadvantageous.

EP-A 0 691 384 discloses aqueous dispersions of dyes prepared using twocopolymers which are likewise prepared by solution polymerization. Thesolvent can be removed later, i.e. before or after the actualapplication of the dispersion. What has been said above applies to theuse of solvents.

Fluorescent dyes are used, for example, to give articles such asplastics or fibers improved whiteness. It is generally desirable to usefluorescent dyes which have a high quantum yield. Oil-solublefluorescent dyes are generally superior to polar and thus water-solublefluorescent dyes. However, it is difficult to use oil-solublefluorescent dyes in processes which take place in an aqueous medium. Ithas hitherto not been possible to use oil-soluble fluorescent dyessuccessfully in paper manufacture, especially in paper coating.

It is an object of the present invention to provide aqueous dispersionsof hydrophobic substances which are simple to prepare and contain a verysmall quantity, if any, of additional emulsifiers. A further object ofthe invention is to provide a simple process for preparing aqueousdispersions of hydrophobic substances. Another object of the inventionis to provide new uses of aqueous dispersions.

We have found that these objects are achieved by the aqueous dispersionsdefined at the outset.

The dispersions of the present invention comprise at least one ethylenecopolymer wax. The ethylene copolymer wax(es) in question is/are atleast one ethylene copolymer wax comprising from 60 to 99.9% by weight,preferably from 65 to 85% by weight, of ethylene and from 0.1 to 40% byweight, preferably from 15 to 35% by weight, of at least oneethylenically unsaturated carboxylic acid in copolymerized form.

The dispersions of the present invention further comprise at least onehydrophobic low molecular weight organic substance. For the purposes ofthe present invention, hydrophobic substances are organic substanceswhich have a solubility of less than 0.5 g/l in water at roomtemperature. For the present purposes, low molecular weight substancesare substances having a molecular weight of up to 2000 g/mol, withpolymer waxes being excluded. In the case of low molecular weightsubstances which have a molecular weight distribution, the molecularweight is taken to be the number average M_(n).

Preferred low molecular weight substances are selected from amongoil-soluble dyes, in particular oil-soluble fluorescent dyes, also knownas optical brighteners, biocides, fragrances and flavors. Preference isgiven to at least one low molecular weight hydrophobic substance presentin the dispersions of the present invention being an oil-soluble dye.

In particular, the low molecular weight hydrophobic substance is anoil-soluble dye. Preferred oil-soluble dyes are those of the class ofoptical brighteners, in particular substituted or unsubstituteddistyrylbiphenyls and distyrylbenzenes. Particular preference is givento

-   -   distyryl compounds of the formula II a    -   benzoxazole derivatives of the formula II b        where

-   R³, R⁴ are identical or different and are selected independently    from among hydrogen, cyano, methyl and ethyl; preference is given to    the two radicals R being identical and the two radicals are    particularly preferably each cyano or methyl;

-   n is an integer in the range from 1 to 5, in particular 1 or 2.

A very particularly preferred example of an oil-soluble dye of theformula II b is the compound of the formula II b.1

The dispersions of the present invention contain from 0.001 to 10% byweight, preferably up to 5% by weight and particularly preferably up to1% by weight, of one or more of the above-described organic lowmolecular weight hydrophobic substances.

Ethylene copolymer waxes comprising ethylene and ethylenicallyunsaturated carboxylic acids can advantageously be prepared byfree-radical-initiated copolymerization under high-pressure conditions,for example in stirred high-pressure autoclaves or in high-pressure tubereactors. Preparation of the copolymer waxes in stirred high-pressureautoclaves is preferred. Stirred high-pressure autoclaves are known perse and a description may be found in Ullmann's Encyclopedia ofIndustrial Chemistry, 5^(th) edition, keywords: waxes, Vol. A 28, p. 146ff., Verlag Chemie Weinheim, Basel, Cambridge, New York, Tokyo, 1996.They usually have a length/diameter ratio in the range from 5:1 to 30:1,preferably from 10:1 to 20:1. The high-pressure tube reactors which canalso be employed are likewise described in Ullmann's Encyclopedia ofIndustrial Chemistry, 5^(th) edition, keywords: waxes, Vol. A 28, p. 146ff., Verlag Chemie Weinheim, Basel, Cambridge, New York, Tokyo, 1996.

Suitable pressure conditions for the polymerization are from 500 to 4000bar, preferably from 1500 to 2500 bar. The reaction temperatures are inthe range from 170 to 300° C., preferably in the range from 200 to 280°C.

The polymerization can be carried out in the presence of one or moreregulators. Regulators used are, for example, hydrogen or an aliphaticaldehyde or an aliphatic ketone of the formula III

or mixtures thereof.

In this formula, the radicals R⁸ and R⁹ are identical or different andare selected from among

-   -   hydrogen;    -   C₁-C₆-alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl,        isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl,        sec-pentyl, neopentyl, 1,2-dimethylpropyl, isoamyl, n-hexyl,        isohexyl, sec-hexyl, particularly preferably C₁-C₄-alkyl such as        methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl        and tert-butyl;    -   C₃-C₁₂-cycloalkyl such as cyclopropyl, cyclobutyl, cyclopentyl,        cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl,        cycloundecyl and cyclododecyl; preference is given to        cyclopentyl, cyclohexyl and cycloheptyl.

In a particular embodiment, the radicals R³ and R⁹ are covalently joinedto one another to form a 4- to 13-membered ring. Thus, R⁸ and R⁹ cantogether be, for example:—(CH₂)₄—, —(CH₂)₅—, —(CH₂)₆, —(CH₂)₇,—CH(CH₃)—CH₂—CH₂—CH(CH₃)— or—CH(CH₃)—CH₂—CH₂—CH₂—CH(CH₃)—.

Well-suited regulators are alkylaromatic compounds, for example toluene,ethylbenzene or one or more isomers of xylene. The use of aldehydes andketones of the formula III as regulators is preferably dispensed with.Preference is given to introducing no further regulators apart from thestabilizers which can be added to make organic peroxides easier tohandle and can likewise have the function of a molecular weightregulator.

As initiators for the free-radical polymerization, it is possible to usethe customary free-radical initiators such as organic peroxides, oxygenor azo compounds. Mixtures of a plurality of free-radical initiators arealso useful.

Suitable peroxides selected from among commercially available substancesare

-   -   didecanoyl peroxide,        2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, tert-amyl        peroxy-2-ethylhexanoate, dibenzoylperoxide, tert-butyl        peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate,        tert-butyl peroxydiethylisobutyrate,        1,4-di(tert-butylperoxycarbo)cyclohexane as an isomer mixture,        tert-butyl perisononanoate,        1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,        1,1-di(tert-butylperoxy)cyclohexane, methyl isobutyl ketone        peroxide, tert-butyl peroxyisopropylcarbonate,        2,2-di(tert-butylperoxy)butane or tert-butyl peroxyacetate;    -   tert-butyl peroxybenzoate, di-tert-amyl peroxide, dicumyl        peroxide, the isomeric di(tert-butylperoxyisopropyl)benzenes,        2,5-dimethyl-2,5-di-tert-butylperoxyhexane, tert-butyl cumyl        peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hex-3-yne,        di-tert-butyl peroxide, 1,3-diisopropylbenzene        monohydroperoxide, cumene hydroperoxide or tert-butyl        hydroperoxide; or dimeric or trimeric ketone peroxides of the        formulae IV a to IV c.

In these formulae, the radicals R¹⁰ to R¹⁵ are identical or differentand are selected from among

-   -   —C₁-C₈-alkyl such as methyl, ethyl, n-propyl, isopropyl,        n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl,        isopentyl, n-hexyl, n-heptyl, n-octyl; preferably linear        C₁-C₆-alkyl such as methyl, ethyl, n-propyl, n-butyl, n-pentyl,        n-hexyl, particularly preferably linear C₁-C₄-alkyl such as        methyl, ethyl, n-propyl and n-butyl, very particularly        preferably ethyl;    -   C₆-C₁₄-aryl such as phenyl, 1-naphthyl, 2-naphthyl, 1-anthryl,        2-anthryl, 9-anthryl, 1-phenanthryl, 2-phenanthryl,        3-phenanthryl, 4-phenanthryl and 9-phenanthryl, preferably        phenyl, 1-naphthyl and 2-naphthyl, particularly preferably        phenyl.

Peroxides of the formulae IV a to IV c and methods of preparing them areknown from EP-A 0 813 550.

Particularly useful peroxides are di-tert-butyl peroxide, tert-butylperoxypivalate, tert-butyl peroxyisononanoate and dibenzoyl peroxide andmixtures thereof. An azo compound which may be mentioned by way ofexample is azobisisobutyronitrile (“AIBN”). Free-radical initiators areintroduced in amounts customary for polymerizations.

Numerous commercially available organic peroxides are admixed withstabilizers before they are sold in order to make them easier to handle.Suitable stabilizers are, for example, white oil and hydrocarbons suchas, in particular, isododecane. Under the conditions of the free-radicalhigh-pressure polymerization, such stabilizers can act as molecularweight regulators. For the purposes of the present invention, referenceto the use of molecular weight regulators means the additional use offurther molecular weight regulators other than the stabilizers.

Monomers used are ethylene together with at least one ethylenicallyunsaturated carboxylic acid. Preference is given to at least oneethylenically unsaturated carboxylic acid being a carboxylic acid of theformula I

In the formula I, the radicals are defined as follows:

-   R¹ is selected from among hydrogen and    -   C₁-C₁₀-alkyl such as methyl, ethyl, n-propyl, isopropyl,        n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl,        sec-pentyl, neopentyl, 1,2-dimethylpropyl, isoamyl, n-hexyl,        isohexyl, sec-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl;        particularly preferably C₁-C₄-alkyl such as methyl, ethyl,        n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and        tert-butyl;-   R² is selected from among hydrogen and    -   C₁-C₁₀-alkyl such as methyl, ethyl, n-propyl, isopropyl,        n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl,        sec-pentyl, neopentyl, 1,2-dimethylpropyl, isoamyl, n-hexyl,        isohexyl, sec-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl;        particularly preferably C₁-C₄-alkyl such as methyl, ethyl,        n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and        tert-butyl;    -   COOH, COOCH₃, COOC₂H₅.

Very particular preference is given to R¹ being hydrogen or methyl andR² being hydrogen.

The ratio in which the monomers are metered in usually does notcorrespond precisely to the ratio of the units in the ethylene copolymerwaxes used according to the present invention because ethylenicallyunsaturated carboxylic acids are generally incorporated more easily intoethylene copolymer waxes than is ethylene.

The monomers are usually metered in jointly or separately.

The monomers can be compressed to the polymerization pressure in acompressor. In another embodiment of the process of the presentinvention, the monomers are firstly brought to an elevated pressure of,for example, from 150 to 400 bar, preferably from 200 to 300 bar and inparticular 250 bar, by means of a pump and then to the actualpolymerization pressure by means of a compressor.

The polymerization can, as a matter of choice, be carried out in theabsence or presence of solvents, with mineral oils, white oil and othersolvents which are present in the reactor during the polymerization andhave been used for stabilizing the free-radical initiator(s) not beingregarded as solvents for the purposes of the present invention.

In one embodiment, the polymerization is carried out in the absence ofsolvents.

The dispersions of the present invention preferably contain from 0.05 to40% by weight, more preferably from 10 to 35% by weight, of one or moreethylene copolymer waxes. It is advantageous for the proportion byweight of the ethylene copolymer wax or waxes in the dispersions of thepresent invention to be not less than that of the hydrophobic lowmolecular weight organic substance or substances. The proportion byweight of the ethylene copolymer wax or waxes in the dispersions of thepresent invention is advantageously more than twice the proportion byweight of the hydrophobic low molecular weight organic substance orsubstances, particularly advantageously more than ten times theproportion by weight of the latter. If the hydrophobic low molecularweight organic substance or substances is/are an oil-soluble fluorescentdye/oil-soluble fluorescent dyes, the proportion by weight of theethylene copolymer wax or waxes in the dispersions of the presentinvention is more than one hundred times the proportion by weight of thehydrophobic low molecular weight organic substance or substances,particularly advantageously more than two hundred times the proportionby weight of the latter.

The dispersions of the present invention usually further comprise one ormore basic substances, for example hydroxides and/or carbonates and/orhydrogencarbonates of alkali metals, ammonia, organic amines such astriethylamine, diethylamine, ethylamine, trimethylamine, dimethylamine,methylamine, ethanolamine, diethanolamine, triethanolamine,methyldiethanolamine, n-butyldiethanolamine, N,N-dimethylethanolamine.The dispersions of the present invention preferably contain an amount ofbasic substance or substances which is such that at least half,preferably at least three quarters, of the carboxyl groups of theethylene copolymer wax or waxes are neutralized.

In one embodiment of the present invention, the dispersions of thepresent invention contain an amount of basic substance or substanceswhich is such that the carboxyl groups of the ethylene copolymer wax orwaxes are quantitatively neutralized.

The dispersions of the present invention usually have a basic pH,preferably a pH of from 7.5 to 14, particularly preferably 8 or aboveand very particularly preferably 8.5 or above.

The dispersions of the present invention preferably do not contain anyemulsifiers or protective colloids. The dispersions of the presentinvention are stable even without such surface active auxiliaries, i.e.at a shear rate of 100 cm⁻¹, the light transmittance alters by no morethan 2%, measured on a dispersion having a solids content of 0.1% byweight and using pure water as reference. Owing to their good useproperties, the dispersions of the present invention have numerousapplications.

The present invention further provides a process for preparing theaqueous dispersions of the present invention. In the process of thepresent invention, one or more ethylene copolymer waxes is/are firstlymixed with at least one hydrophobic low molecular weight organicsubstance and the mixture is subsequently dispersed in water.

The procedure for carrying out the process of the present inventionstarts out from one or more of the above-described ethylene copolymerwaxes. This is placed in a vessel, for example a flask, an autoclave ora batch reactor. In one variant, one or more hydrophobic low molecularweight organic substances are added and the ethylene copolymer wax orwaxes is heated to a temperature above its melting point. It isadvantageously heated to a temperature which is at least 10° C.,particularly advantageously at least 30° C., above the melting point ofthe ethylene copolymer wax or waxes.

If a plurality of different ethylene copolymer waxes are used, themixture is heated to a temperature which is above the melting point ofthe ethylene copolymer wax having the highest melting point. When aplurality of different ethylene copolymer waxes are used, the mixture isadvantageously heated to a temperature which is at least 10° C. abovethe melting point of the ethylene copolymer wax having the highestmelting point. When a plurality of different ethylene copolymer waxesare used, the mixture is particularly advantageously heated to atemperature which is at least 30° C. above the melting point of theethylene copolymer wax having the highest melting point.

Dispersion of the hydrophobic low molecular weight organic substance orsubstances can be aided by further measures, for example by mechanicalor pneumatic stirring or by shaking.

In one variant of the process of the present invention, the ethylenecopolymer wax or waxes is firstly heated and the hydrophobic lowmolecular weight organic substance or substances is/are added to themolten ethylene copolymer wax or waxes. It is also possible for thehydrophobic low molecular weight organic substance or substances to beadded a little at a time during the heating procedure.

Water and one or more basic substances and, if desired, furtherconstituents such as ethylene glycol are subsequently added. The orderof the addition of water and the addition of the basic substance orsubstances and further constituents is immaterial. If the temperature isabove 100° C., it is advantageous to work under superatmosphericpressure and to choose the vessel correspondingly. The emulsion formedis homogenized, for example by mechanical or pneumatic stirring or byshaking. The aqueous dispersion prepared in this way can subsequently becooled.

The present invention further provides for the use of the aqueousdispersions of the present invention in paper coating.

The present invention further provides paper coating compositionscomprising the dispersions according to the present invention andprovides a process for preparing the paper coating compositions of thepresent invention using the dispersions of the present invention.

The paper coating compositions of the present invention further comprisecustomarily used pigments, in particular white pigments, for examplebarium sulfate, calcium carbonate, calcium sulfoaluminate, kaolin, talc,titanium dioxide, chalk or coating clay. Pulverulent plastics, forexample polystyrene, are also suitable.

The paper coating compositions of the present invention comprise atleast one pigment. The other constituents of the paper coatingcompositions of the present invention are typically standardized on thebasis of the proportion of pigment.

In one embodiment of the present invention, the paper coatingcompositions of the present invention comprise one or more dispersionsaccording to the present invention in a proportion of from 0.5 to 95% byweight, based on the pigment or pigments; preference is given to from 1to 50% by weight.

The paper coating compositions of the present invention can furthercomprise additional dispersants which can serve to disperse the pigmentor pigments. Suitable dispersants are, for example, aqueous polymersolutions. Examples of aqueous polymer solutions include: aqueoussolutions of polyalkali metal salts of poly(meth)acrylic acid,copolymers of (meth)acrylic acid and C₁-C₁₀-alkyl (meth)acrylates, withthe copolymers being able to be fully or partially neutralized by basicalkali metal salts.

The concentrations of the aqueous polymer solutions are usually in therange from 10 to 50% by weight. If the use of dispersants is desired,amounts of, for example, from 0.01 to 5% by weight, based on the pigmentor pigments, are useful.

The paper coating compositions of the present invention can furthercomprise one or more binders. Suitable binders are, for example, aqueouspolymer dispersions of polymers or copolymers of one or more of thefollowing monomers: butadiene, styrene, acrylonitrile, vinyl acetate,C₁-C₁₀-alkyl (meth)acrylates, hydroxy-C₁-C₁₀-alkyl (meth)acrylates,acrylamide and N-methylolacrylamide. The proportion of solids is usuallyin the range from 30 to 70% by weight, preferably from 40 to 60% byweight. Examples of polymer dispersions which can be added as binder tothe paper coating compositions of the present invention are described inU.S. Pat. No. 3,404,116 and U.S. Pat. No. 3,990,080. If the addition ofa binder is desired, typical proportions are in the range from 0.1 to50% by weight, preferably from 2 to 20% by weight, particularlypreferably from 8 to 14% by weight, of aqueous polymer dispersion, basedon the pigment or pigments, as binder.

If the use of relatively large amounts of dispersions according to thepresent invention, for example in the range from 60 to 95% by weightbased on the pigment or pigments, is desired, it is possible to addsmaller proportions of binder, for example in the range from 0.1 to 14%by weight based on the pigment or pigments, or to leave out the binderor binders entirely.

The paper coating compositions of the present invention can furthercomprise cobinders. Examples of natural cobinders are starch, casein,gelatin, alginates and soya protein. Examples of modified naturalcobinders are hydroxyethylcellulose, methylcellulose andcarboxymethylcellulose and also cationically modified starch. Examplesof synthetic cobinders are the customary synthetic cobinders, forexample cobinders based on polyvinyl acetate or polyacrylate.

Cobinders can be present in a proportion of from 0.1 to 10% by weight,based on the pigment content.

A further constituent of the paper coating compositions of the presentinvention is water. The water content is usually set to from 25 to 75%by weight, based on the total mass of the paper coating composition ofthe present invention.

To produce a paper coating composition according to the presentinvention, it is possible to employ methods known per se.

The paper coating compositions of the present invention can be appliedto various materials, in particular to paper. They are applied using thedevices customarily employed, for example doctor blades.

The present invention further provides paper containing oil-solublefluorescent dyes. It displays a particularly advantageous whiteness andcan be produced using small amounts of fluorescent dyes having a highquantum yield. It has overall advantageous use properties. The contentof oil-soluble fluorescent dyes in the paper according to the presentinvention is generally from 0.0001 to 5% by weight, preferably from0.001 to 3% by weight, particularly preferably from 0.002 to 1% byweight, in each case based on paper.

A specific aspect of the present invention is paper treated with thepaper coating compositions according to the present invention. Paperaccording to the present invention has a particularly advantageouswhiteness and can be produced using small amounts of fluorescent dyeshaving a high quantum yield. It has overall advantageous use properties.The content of oil-soluble fluorescent dyes in the paper coatedaccording to the present invention is, in particular, from 0.0001 to 5%by weight, preferably from 0.001 to 3% by weight, particularlypreferably from 0.002 to 1% by weight.

The invention is illustrated by working examples.

WORKING EXAMPLES

1. Preparation of Ethylene Copolymer Wax

Ethylene and methacrylic acid were copolymerized in a high-pressureautoclave as described in the literature (M. Buback et al., Chem. Ing.Tech. 1994, 66, 510). For this purpose, ethylene (12.3 kg/h) was fedinto the autoclave under the reaction pressure of 1700 bar. Separatelytherefrom, 1.04 l/h of methacrylic acid were firstly compressed to anintermediate pressure of 250 bar and subsequently fed in under thereaction pressure of 1700 bar. Separately therefrom, 2 l/h of aninitiator solution comprising tert-butyl peroxypivalate (0.13 mol·l⁻¹ inisododecane) were fed into the autoclave under the reaction pressure of1700 bar. The reaction temperature was 220° C. This gave 3.4 kg/h ofethylene copolymer wax having the following properties: 26% by weight ofmethacrylic acid, 76% by weight of ethylene, melting range: 70-80° C.,dynamic melt viscosity η: 68 000 mPa·s, measured at 120° C. inaccordance with DIN 51562, acid number: 172 mg KOH/g (determined inaccordance with DIN 53402).

The contents of ethylene and methacrylic acid in the ethylene copolymerwax were determined by NMR spectroscopy and by titration (acid number).The acid number of the ethylene copolymer wax was determinedtitrimetrically in accordance with DIN 53402. The KOH consumptioncorresponds to the methacrylic acid content of the ethylene copolymerwax.

2. Preparation of a Dispersion According to the Present Invention

200 g of ethylene copolymer wax prepared as described in Example 1 wereplaced in a 2 liter stirred vessel provided with an anchor stirrer andreflux condenser. 1 g of the fluorescent dye of the formula II a.1

was added and the mixture was heated to 160° C. while stirring. Stirringwas continued for another one hour. The fluorescent dye dissolvedcompletely. The mixture was cooled to 140° C., a pH of 8 was set usingN,N-dimethylethanolamine and the mixture was made up with water to atotal volume of 1 liter. The solids content was 19.7% by weight.3. Production and Use of Paper Coating Compositions

In a flask provided with a stirrer, the following were mixed with oneanother:

-   253 g of a dispersion according to the present invention from    Example 2,-   100 g of calcium carbonate (90% by weight of the particles having a    diameter of less than 2 μm, commercially available as Hydrocarb 90®    from Omya),-   0.3 g of sodium polyacrylate, M_(w)=4000 g, fully neutralized,-   0.2 ml of a 25% strength by weight aqueous solution of NaOH and-   0.75 g of an aqueous dispersion of an acrylic acid-acrylic ester    copolymer having the following characteristics: acrylic acid 44% by    weight, ethyl acrylate 56% by weight, Brookfield viscosity at 100    s⁻¹ of a 1% strength by weight solution at a pH of 9.5: 15 mPa·s,    solids content of the dispersion: 39.7% by weight. The mixture    obtained in this way was adjusted to a solids content of 39.5% by    weight by addition of water. This gave a paper coating composition    having a pH of 9.1. The paper coating composition obtainable in this    way was applied by means of a hand doctor blade to uncoated    Scheufelen paper (coating weight: 12 g/m²) and dried.

The whiteness (R457) was determined as 91.5% at 457 nm by means of anElrepho G-50-660 spectrometer in accordance with DIN 53 145 using UVlight; without UV light, a value of 83.5% at 457 nm was found. This gavea difference of 8.0%.

1. An aqueous dispersion comprising at least one ethylene copolymer waxcomprising from 60 to 99.5% by weight of ethylene and from 0.5 to 40% byweight of at least one ethylenically unsaturated carboxylic acid incopolymerized form and also at least one hydrophobic low molecularweight organic substance.
 2. The dispersion as claimed in claim 1,wherein at least one ethylenically unsaturated carboxylic acid is acarboxylic acid of the formula I

in which the radicals are defined as follows: R¹ is selected from amonghydrogen and unbranched or branched C₁-C₁₀-alkyl, R² is selected fromamong hydrogen, unbranched or branched C₁-C₁₀-alkyl and COOH, COOCH₃,COOC₂H₅.
 3. The dispersion as claimed in claim 1, wherein at least onelow molecular weight hydrophobic substance is an oil-soluble dye.
 4. Thedispersion as claimed in claim 1, wherein the low molecular weighthydrophobic substance is selected from among distyryl compounds andbenzoxazole derivatives.
 5. A process for preparing aqueous dispersionsas claimed in claim 1, which comprises firstly mixing one or moreethylene copolymer waxes with at least one hydrophobic low molecularweight organic substance and subsequently dispersing the mixture inwater.
 6. The use of dispersions as claimed in claim 1 in paper coating.7. A paper coating composition comprising a dispersion as claimed inclaim
 1. 8. (canceled)
 9. A Paper treated with a paper coatingcomposition as claimed in claim
 7. 10. The dispersion as claimed inclaim 2, wherein at least one low molecular weight hydrophobic substanceis an oil-soluble dye.
 11. The dispersion as claimed in claim 2, whereinthe low molecular weight hydrophobic substance is selected from amongdistyryl compounds and benzoxazole derivatives.
 12. The dispersion asclaimed in claim 3, wherein the low molecular weight hydrophobicsubstance is selected from among distyryl compounds and benzoxazolederivatives.
 13. The process for preparing aqueous dispersions asclaimed in claim 2, which comprises firstly mixing one or more ethylenecopolymer waxes with at least one hydrophobic low molecular weightorganic substance and subsequently dispersing the mixture in water. 14.The process for preparing aqueous dispersions as claimed in claim 3,which comprises firstly mixing one or more ethylene copolymer waxes withat least one hydrophobic low molecular weight organic substance andsubsequently dispersing the mixture in water.
 15. The process forpreparing aqueous dispersions as claimed in claim 4, which comprisesfirstly mixing one or more ethylene copolymer waxes with at least onehydrophobic low molecular weight organic substance and subsequentlydispersing the mixture in water.
 16. The use of dispersions as claimedin claim 2 in paper coating.
 17. The use of dispersions as claimed inclaim 3 in paper coating.
 18. The use of dispersions as claimed in claim4 in paper coating.
 19. The use of dispersions as claimed in claim 5 inpaper coating.
 20. A paper coating composition comprising a dispersionas claimed in claim
 2. 21. A paper coating composition comprising adispersion as claimed in claim
 3. 22. A paper coating compositioncomprising a dispersion as claimed in claim
 4. 23. A paper coatingcomposition comprising a dispersion as claimed in claim
 5. 24. A papercoating composition comprising a dispersion as claimed in claim 6.